1. Field of the Invention
The present invention relates to a driving circuit and method of an active matrix organic light-emitting device (AMOLED), and more particularly to a driving technique that uses the power pulse feed-through technique to stabilize the current flowing through the light-emitting device.
2. Description of Related Art
There are many types of flat panel display in the market, such as LCD, PDP and OLED etc. At present, the OLED products still suffer from many technique problems needed to be solved. For example, a driving voltage (VOLED) is dropped on the organic light-emitting device when the organic light-emitting device is driven by the driving circuit. The driving voltage (VOLED) is gradually increased with time to unsteady the driving current during the organic light-emitting device is driven, since the material characterization of the organic light-emitting device. In addition, the threshold voltage of a driving transistor in driving circuit has similar material problem. The threshold voltage is increased with time when the driving transistor is driven for a long time. The increasing threshold voltage unsteadies the driving current to affect the image quality of the organic light-emitting device.
With reference to FIG. 6, a typical OLED driving circuit with a 2T-1C configuration includes a switching transistor (M1), a driving transistor (M2), and a storage capacitor (Cs). The conventional driving circuit is also disclosed in prior art of the U.S. Pat. No. 6,680,580 (hereinafter '580) and U.S. Pat. No. 6,677,713 (hereinafter '713). A gate terminal (G) of the switching transistor (M1) is connected to a scan line to receive a scanning signal (Vscan), a drain terminal (D) of the switching transistor (M1) is connected to a data line to receive an image data signal (Vdata), and a source terminal (S) is connected to a gate terminal (G) of the driving transistor (M2) to control ON/OFF states of the driving transistor (M2). If the driving transistor (M2) is an n-channel type transistor, its drain terminal (D) is connected to a high or positive voltage source (VDD) and its source terminal (S) is connected to an anode of the organic light-emitting device (OLED). The cathode of the organic light-emitting device (OLED) is connected to a low or negative voltage source (VSS). The storage capacitor (Cs) is connected between the gate terminal (G) of the driving transistor (M2) and a reference voltage (Vref). The storage capacitor (Cs) can assist the driving transistor (M2) to be kept in either the ON or OFF states.
When the gate terminal (G) of the switching transistor (M1) receives the scanning signal (Vscan) provided by the scan line, the image data signal (Vdata) is transmitted to the gate terminal (G) of the driving transistor (M2) and the storage capacitor (Cs). If the voltage of the image data signal (Vdata) is larger than a threshold voltage (Vth) of the driving transistor (M2), the driving transistor (M2) will become conducted to allow a driving current (ID2) to activate the light-emitting device.
However, with reference to FIG. 7 and FIG. 8, if the organic light-emitting device (OLED) has been operated for a long time, the OLED driving voltage (VOLED) may gradually increase which results in a reduction in the driving current (ID2). As a result, the brightness of the organic light-emitting device (OLED) weakens. Equations with regard to the driving current (ID2) in the conductive condition are shown to explain the relationship between the OLED driving voltage (VOLED) and the brightness of the organic light-emitting device (OLED).
            I              D        ⁢                                  ⁢        2              =                  1        2            ⁢      μ      ⁢                          ⁢              C        OX            ⁢              W        L            ⁢                        (                                    V                              GS                ⁢                                                                  ⁢                2                                      -                          V                              th                ⁢                                                                  ⁢                2                                              )                2                        I              D        ⁢                                  ⁢        2              =                  1        2            ⁢      μ      ⁢                          ⁢              C        OX            ⁢              W        L            ⁢                        (                                    V                              G                ⁢                                                                  ⁢                2                                      -                          V                              S                ⁢                                                                  ⁢                2                                      -                          V                              th                ⁢                                                                  ⁢                2                                              )                2                        where      ⁢                          ⁢              V                  S          ⁢                                          ⁢          2                      =                  V        OLED            +              V        SS                        I              D        ⁢                                  ⁢        2              =                  1        2            ⁢      μ      ⁢                          ⁢              C        OX            ⁢              W        L            ⁢                        (                                    V                              G                ⁢                                                                  ⁢                2                                      -                          V              OLED                        -                          V              SS                        -                          V                              th                ⁢                                                                  ⁢                2                                              )                2            
Based on the above equations, the decrease in driving current (ID2) occurs when the OLED driving voltage (VOLED) increases. The OLED driving voltage (VOLED) of the organic light-emitting device (OLED) increases with time while the driving current (ID2) decreases with time. In addition, after supplying the positive voltage to the gate and source terminals (G, S) of the driving transistor (M2) for a long time, the threshold voltage (Vth) is also increased with further reference to FIG. 9.
Based on foregoing description, an unstable voltage of the organic light-emitting device (OLED) and a variable threshold voltage (Vth) of the driving transistor (M2) will reduce the brightness of the organic light-emitting device (OLED).
Therefore, the image display of the organic light-emitting device is not good after driving for a long time. To improve material fault of the organic light-emitting device and the driving transistor, many flat panel display factories accordingly propose many modified driving circuits to overcome the fault to improve the display quality of the OLED product.
With reference to FIG. 10, the same with FIG. 4 of the '713 patent, an OLED driving circuit with a 3T1C configuration is disclosed to maintain the threshold voltage (Vth) of a driving transistor (M2) at a stable value after long operation time of image display. The driving circuit of the '713 patent is formed by incorporating a 2T1C driving unit with an additional switching transistor (M3). A gate terminal (G) of the additional switching transistor (M3) is connected to another scan line, a drain terminal (D) thereof is connected to the gate terminal (G) of the driving transistor (M2) of the 2T1C driving unit, and a source terminal (S) is connected to another reference voltage source (Vref2) with a low voltage. With further reference to FIG. 11, there are two pulse signals (VscanA and VscanB) to be supplied to the two scan lines respectively. The two pulse signals have the same frequency and a delay time exists there between. When the two pulse signals (VscanA and VscanB) are supplied to the two scan lines respectively, the two switching transistors (M1, M3) will be activated alternately. Therefore, the gate terminal (G) of the driving transistor (M2) receives alternate high/low voltages. Regarding to the low voltage supplied to the gate terminal (G) of the driving transistor (M2), the driving transistor (M2) will be turned off to stop the driving current (ID) to the organic light-emitting device (OLED), so we called this condition as Negative bias annealing technique. Since the driving transistor (M2) is alternately controlled in ON/OFF states, the variable threshold voltage (Vth) of the driving transistor (M2) can be solved. However, since this 3T1C driving circuit uses one additional switching transistor (M3), another scan line and reference voltage (Vref2) are required. Not only the aperture ratio of each pixel of the OLED product will be decreased, but also the layout of an extra control lines are more complex. In addition, the 3T1C driving circuit does not make the voltage of the organic light-emitting device (OLED) in a stable value. Therefore, the brightness of the organic light-emitting device is (OLED) still decreasing with time.
With reference to FIG. 12, the same with the FIG. 4 of the '580 patent, another 3T1C configuration of an OLED driving circuit is disclosed to maintain the driving voltage of the organic light-emitting device (OLED) on a stable value even under a long time operation of displaying image. The driving circuit has a first switching transistor (M1), a driving transistor (M2), a storage capacitor (CS) and a second switching transistor (M3). Two gate terminals (G) of the first and second switching transistors (M1, M3) are connected to the same scan line (Vscan). The two source terminals (S) of the first switching transistor and driving transistor (M1, M2) are respectively connected to the two ends of the storage capacitor (Cs). The drain terminal (D) of the first switching transistor (M1) is connected to the data line (Vdata). The drain terminal (D) of the driving transistor (M2) is connected to the high or positive voltage (VDD). The gate terminal (G) of the driving transistor (M2) is connected to the source terminal (S) of the first switching transistor (M1). The drain terminal (D) of the second switching transistor (M3) is connected to the source terminal (S) of the driving transistor (M2).
The source terminal (S) of the driving transistor (M2) is further connected to an anode of the organic light-emitting device (OLED) and a cathode of the organic light-emitting device (OLED) is connected to a low or negative voltage source (VSS).
The second switching transistor (M3) is connected between the source terminal (S) of the driving transistor (M2) and a common voltage (Vcom). Therefore, when the first and second switching transistors (M1, M3) are all activated, the common voltage (Vcom) is directly supplied to the source terminal (S) of the driving transistor (M2). That is, the voltage of the source terminal (S) of the driving transistor (M2) does not change according to the variable driving voltage (VOLED) of the organic light-emitting device (OLED). Thus, the driving current (ID) is represented as follow:Vg=Vdata Vs=Vcom
      I    D    =            1      2        ⁢    μ    ⁢                  ⁢          C      OX        ⁢          W      L        ⁢                  (                              V            data                    -                      V            com                    -                      V            th                          )            2      
The driving current (ID) can be maintained in a stable value. With further reference to FIG. 13, the '580 patent uses a pulse signal as a frame signal, wherein the pulse is consisted of one purposely-interleaved frame (OFF) between two original frames (ON), to practice negative bias annealing technique to keep the threshold voltage (Vth) of the driving transistor (M2) in a stable value. In nth frame, since the frame state is at a high level (ON), the image data signal (Vdata) is high and supplied to the gate terminal (G) of the driving transistor (M2). At the same time, the first and second switching transistors (M1, M3) are conductive when the voltage (Vscan) of the scan line is high. Meanwhile, the source terminal (S) of the driving transistor (M2) obtains the common voltage (Vcom) through the conductive second switching transistor (M3) (Vs=Vcom). The image data signal (Vdata) will be supplied to the gate terminal (G) of the driving transistor (M2) (Vg=Vdata). Further, the common voltage (Vcom) is smaller than the voltage of the image data signal (Vdata). Therefore, the voltage of the image data signal (Vdata) subtracts the common voltage (Vcom) to have the potential between the gate and source terminals (G, S) of the driving transistor (M2) (VGS=Vdata−Vcom). Since the driving transistor (M2) obtains a bias voltage, the driving current (ID) passes through the organic light-emitting device (OLED). In next frame, the frame state is low level to make the voltage of the image data signal (Vdata) being lower than the common voltage (Vcom). Therefore, the driving transistor (M2) is not conductive to complete the negative bias annealing technique.
Although the driving circuit of '580 patent can avoid the change in the driving current (ID) resulted from the increased voltage of the organic light-emitting device (OLED) and maintain the threshold voltage (Vth) of the driving transistor (M2) in a stable value, the driving circuit still has the drawbacks as follow:
1. The driving current (ID) through the organic light-emitting device (OLED) is decreased, the brightness of the organic light-emitting device (OLED) weakens accordingly. When the voltage (Vscan) of the scan line is high, the first and second switching transistors (M1, M3) are conductive and the gate voltage of the driving transistor (M2) is equal to the voltage (Vg) of the data line. Then, the driving transistor (M2) and the second switching transistor (M3) are conductive. The conductive driving and second switching transistors (M2, M3) respectively have an inner resistance (RM2) (RM3), so the voltage (VS) of the source terminal of the driving transistor (M2) is represented by
            R              M        ⁢                                  ⁢        3                            R                  M          ⁢                                          ⁢          2                    +              R                  M          ⁢                                          ⁢          3                      ×            (                        V          DD                -                  V          COM                    )        .  Therefore, the voltage (VS) of the source terminal of the driving transistor (M2) is not equal to the common voltage (Vcom).
2. The '580 patent uses a pulse signal as a frame signal, wherein the pulse is consisted of one purposely-interleaved frame (OFF) between two original frames (ON) to practice negative bias annealing technique to maintain the threshold voltage (Vth) in a stable value. Therefore, the original frame is shortened, as a result, the image display quality of the OLED product is affected.
With reference to FIG. 14, another 3T1C driving circuit disclosed by Li in the U.S. Pat. No. 6,756,741 (hereinafter '741) has a first and second switching transistors (M1, M2), a driving transistor (M3) and a storage capacitor (CS). Two gate terminals (G) of the first and second switching transistors (M1, M2) are connected to the same scan line. Two drain terminals (D) of the second switching transistor (M2) and the driving transistor (M3) are connected to the high or positive voltage source (VDD). A source terminal (S) of the second switching transistor (M2) and a gate terminal (G) of the driving transistor (M3) are connected to one end of the storage capacitor (CS). A drain terminal (D) of the first switching transistor (M1) and a source terminal (S) of the driving transistor (M2) are connected to the other end of the storage capacitor (CS) and an anode of the organic light-emitting device (OLED). A cathode of the organic light-emitting device (OLED) is connected to ground.
When the scan line has a high voltage, the first and second switching transistors (M1, M2) are turned on. At the time, the two ends of the storage capacitor (CS) respectively obtain a voltage of the image data signal (Vdata) and a high voltage source (VDD). The potential between the gate and source terminals (G, S) of the driving transistor (M3) can be driven by subtracting the voltage of the image data signal from the high voltage source (VGS=VDD−Vdata). The bias voltage of the driving transistor (M3) is affected by the voltage of the organic light-emitting device (OLED). However, the voltage over the storage capacitor (CS) is not equal to the voltage of the image data signal (Vdata) to generate a static current, since the first switching transistor (M1) and the organic light-emitting device (OLED) are resistance elements. Therefore, quality of the image display is worse than that of the foregoing mentioned 3T1C driving circuits in '580 and '713 patents. In addition, the '741 patent still has the problem of variable threshold voltage.
Therefore, the present invention provides a new 3T1C driving circuit for AMOLED product to overcome the material faults of organic light-emitting device and the driving transistor caused unstable driving current.